1. Field of the Invention
The present invention relates generally to a method and apparatus for establishing a selected valid data volume preferably in the cross-sectional form of a diamond shape from which data is accepted. More specifically, the invention relates to a method and apparatus for illuminating particles suspended in a gas stream moving transversely through a radiation beam by generating two signals which are a function of the position of the particles and accepting only data for the particles producing preselected relative signal characteristics indicative of particle location in the desired volume.
2. Description of the Prior Art
Measurement of characteristics of particles suspended in a fluid stream, such as an air stream, by moving the fluid stream through a beam of radiation is known. However, spurious results are all too easily obtained unless the conditions of measurement are carefully controlled. For instance, a beam of coherent radiation varies substantially in intensity acorss the width thereof. Accordingly, if the brightness, transmission, backscatter, etc., of the beam with regard to an observed particle is a measured parameter, it is important that the measurement be made in a narrowly controlled portion of the beam. Similarly, the influence of a conduit directing a fluid stream through a beam of radiation is more significant adjacent the walls of the conduit. Accordingly, it is desirable that only a selected portion of the fluid stream be utilized for measurement.
Mechanical means can be utilized to limit the volumes from which data is accepted. With respect to particle measurement apparatus, a conduit can be employed to guide the particle only from the center of the beam. The disadvantage of this approach lies in the fact that such conduits themselves influence flow of particles, particularly when the particles are of substantial size relative to the beam, and, thus, introduce an uncertainty into the results.
Alternatively, the acceptability of data can be determined upon a statistical basis to discriminate against datum which, based upon certain measured parameters, is typical of events outside of the valid sample volume. However, such statistical approaches are relatively complicated and, not being a direct control, can exclude unusual but valid data.